Conventionally, various forms of scanning devices for scanning a detecting light beam are known, such as scan type laser radars, laser scanners, laser printers, laser markers and object monitoring devices. Among such devices, actuators for scanning a detecting light beam used in scan type laser radars for preventing a vehicle crash include those impinging a light beam onto a point on a polygonal mirror which is rotated by a motor and using the light beam reflected by the polygon mirror as a detecting light beam (FIG. 21), and those using a single moveable mirror turned or swung by a motor to reflect a light beam impinged thereonto from a laser light source to scan the light beam by reflection as a detecting light beam (FIG. 22) (see Japanese patent laid open publication No. 03-175390 and Japanese patent laid open publication No. 07-92270).
The polygon mirror type device illustrated in FIG. 21 includes a polygon mirror 31 which is rotatively driven by an electric motor 32, and a fixed reflective mirror 34 which directs a laser beam LB emitted from a laser diode 33 onto a point on the polygon mirror 31 so that the reflected laser beam may be scanned as the different reflective surfaces of the polygon mirror pass this point.
Such a polygon mirror type device is capable of a high speed scanning, but is both high is cost and large in size because a bearing is required for a sliding part that rotatably supports the mirror as well as an electric motor for swinging or rotating the mirror.
The single mirror type device illustrated in FIG. 22 includes a single moveable mirror 35 which is cyclically swung by an electric motor 36. Laser light emitted from a laser diode 37 is impinged upon the moveable mirror 35 to scan the laser beam LB reflected by the moveable mirror. In this case, the moveable mirror is typically swung in a cyclic manner by using a cam driven by a motor.
Such a single mirror type device is suitable for compact design, and costs less than a polygon mirror type device, but the need for a bearing for rotatably supporting the mirror and an electric motor for driving the mirror thereof prevents a reduction in cost. In particular, the need to rotate a single mirror prevents an increase in the scanning speed. and the need to swing a single, mirror prevents achievement of both compact design and high frequency drive because of the problems associated with inertia and drive torque.